Abstract: A method and apparatus are provided for initializing a closed-loop fiber optic gyroscope system of the type that includes a fiber optic gyroscope for generating an interferometric measurement of phase shift and a demodulator for converting the gyroscope output to a corresponding value in a predetermined format for application to a feedback integrator. In contrast to prior art methods and apparatus that require recapture, the present invention relies upon the concept of resetting the initial value received from the gyro to a corresponding value that lies within the zeroeth fringe of a gyro output. The feedback integrator includes an arithmetic unit with feedback. The resetting process consists of the performance of an operation upon the value. Such operations include shifting and copying functions when the predetermined format is two's-complement logic, integer function when the predetermined format is fixed point logic and both INTEGER and ROUND functions when the predetermined format is floating point logic.

Abstract: The invention is a method for improving the performance of a fiber-optic gyro comprising counter-propagating light beams from a broadband source in an optical fiber configured as a coil. The fiber-optic gyro is of the type where the phases of the light beams are step-modulated at one or both ends of the optical fiber at time intervals equal to the light propagation time .tau. through the optical fiber, the step being in the range from 0 to 2.pi.-.delta. radians where .delta. is an infinitesimal. The step is the sum of an estimate of the Sagnac phase residual .phi..sub.SE with reversed sign and the quantity (2.pi.k+j.phi..sub.M) where the index k can take on the values -1, 0, and 1, and the index j can take on the values -1 and 1. The phase angle .phi..sub.M is a positive constant less than .pi. radians. The method comprises the steps of determining the interferometer output light intensity, determining the Sagnac fringe number n, calculating the error P.sub.

Abstract: A gyroscope assembly (10) includes a ring-shaped fiber optic coil (14) and a coil conforming enclosure (12, 16) of high magnetic permeability ferromagnetic material. The enclosure is ring-shaped to conform with the shape of the coil, and includes a portion (22) extending within the internal hole of the coil ring. Therefore, the coil is intimately and fully encased within high magnetic permeability material. In particular, the enclosure comprises a coil supporting spool (12) and a cover (16) secured to the spool. The spool includes a base (18) which is provided with a central hole (20) and a tubular wall (22) extending perpendicularly from the base. Coil (14) is bonded to base (18). Both the spool and the cover are formed of high magnetic permeability material, and the cover is placed about the fiber optic coil and attached to the spool. The coefficient of thermal expansion material used for the spool is matched to that of the coil pack to minimize stress imposed upon the fiber.

Abstract: The invention is a method and apparatus for compensating for sculling in a strapdown inertial navigation system. The method operates on a sequence of inputs .DELTA.V.sub.B (n) derived at times n.DELTA.t from the outputs of one or more accelerometers. Separately compensated quantities .DELTA.V.sub.Bc (m,p) calculated at times (pJ+1/2).DELTA.t for M values of m are obtained by summing A(m,k).DELTA.V.sub.B (pJ-j-k) over k from 0 to K-1 and over j from 0 to J-1 where the constants A(m,k) are chosen to minimize sculling error in the individual .DELTA.V.sub.Nc (m,p) quantities where .DELTA.V.sub.Nc (m,p) is the representation of .DELTA.V.sub.Bc (m,p) in the navigation frame of reference. The final compensated quantity .DELTA.V.sub.Bc (p) is obtained by summing B(m).DELTA.V.sub.Bc (m,p) over m where the constants B(m) are chosen to minimize sculling error in .DELTA.V.sub.Nc (p) where .DELTA.V.sub.Nc (p) is the representation of .DELTA.V.sub.Bc (p) in the navigation frame of reference.

Abstract: A closed-loop system for regulating a fiber optic gyroscope is configured so that the residual error of a so-called deadbeat system assumes a higher order function of frequency. The closed-loop gyro includes a rate controller within the feedback path. The rate controller receives the gyro output from the last loop transit corrected for d.c. offset and for nominal dither and outputs a gyro rate estimate. The rate controller includes a plurality of feedback integrators in cascaded relationship. In this way, the residual gyro error associated with inherent loop delay is elevated to a higher-order relationship to sensed gyro rate. As a result, a substantial reduction in residual error as a function of sensed gyro rate occurs throughout the range of operational gyro frequencies.

Abstract: The invention is a method for compensating for coning in a strapdown inertial navigation system which utilizes groups of five successive incremental angles of rotation of a body-fixed coordinate system as measured by orthogonally-mounted gyros at regular measurement intervals, each group of five measurements being obtained during a group interval equal to five measurement intervals. The coning-compensated angular displacement of the body-fixed coordinate system about a fixed axis in space during a p'th group interval is obtained by summing the five measured incremental angles and a coning compensation term.

Abstract: A gyroscope component (10) and its variations (10a, 10b and 10c) includes a spool (12, 12a) which supports a ring-shaped fiber optic gyroscope coil (14). The spool has a base (26, 26a) which is provided with a central hole (38, 38a). A perpendicularly extending tubular wall (28, 28a) extends from the base or a cover (16, 16a), and may directly contact the coil or be separated therefrom by a non-adhesive material (29) or a space (30). The cover covers the spools and, with the spool and the tubular wall, forms a ring-shaped enclosure. Each coil has a generally trapezoidal shape and comprises an optical fiber winding wound about an axis and having a cross-sectional configuration that approximates a 45.degree. right triangle whose hypotenuse is formed by the sloped sides to provide a side which is sloped with respect to its axis. Three components (10, 10", 10"') are positioned respectively about three orthogonally disposed axes to form an assembly (50) and are supported within and enclosed by a housing (52).

Abstract: A ring laser gyroscope having an alternating Faraday bias includes mode combining apparatus arranged to cause interference between the modes that propagate within the cavity. Signal separating apparatus separates a signal indicative of rotation of the frame from other components of the combined intensity signal. The signal separating apparatus produces two signals indicative of the light intensity in the interference pattern of the combined modes. These two signals have a 180.degree. phase difference in corresponding components that are dependent upon the geometric bias caused by the non-planar optical path to produce a rotation signal that is independent of the spatially uniform interference fringes caused by the geometric bias.

Abstract: A loop controller for a fiber optic gyroscope employs distributed data processing to compress numerous additional useful functions into each loop transit time. The architecture of the loop controller includes a field programmable gate array for generating variables of varying sign. In addition, the array acts to buffer the transmission of parameters between the gyro processor and an auxiliary processor. The auxiliary processor updates such parameters (which do not require updating every loop transit time) and a number of such parameters are multiplied by values generated by the array in operations performed by the gyro processor that replace what would otherwise require throughput intensive test and branch operations. The combination of the off-loading of selected variable and parameter generation and updating with programming of the gyro processor for parallel processing permits the loop controller to perform numerous useful operations not previously possible during a realistic loop transit time.

Abstract: A phase modulator applies sequences of artificial phase shifts in the region of overmodulation between light beams counterpropagating within the sensor coil of a Sagnac interferometer to obtain improved random walk performance. The sequences are composed of phase shifts of .+-.a.pi./2b and .+-.(4b-a).pi./2b where a is an odd integer and b is an integer greater than 1. The waveform for driving the phase modulator is derived from a 2.pi. radian mapping plotted so that adjacent driving signal values differ by a.pi./2b or -(4b-a).pi./2b in a first angular direction and by -a.pi./2b or by (4b-a).pi./2b in the opposite angular direction. Angular transitions about the mapping may be predetermined to generate a deterministic, random, or pseudo-random modulation sequence. In the event that a deterministic sequence is chosen, it is examined for orthogonality properties to assume zero average output bias from crosstalk.

Abstract: A digital circuit for adapting the output of a closed-loop accelerometer to a utilization circuit where the cycle time of the rebalance loop of the accelerometer is unrelated to the cycle time of the utilization circuit. A register receives and stores a digital value representative of measured acceleration and a bank of serially-arranged divide-by-two counters accepts the output of the rebalance loop clock. A gate arrangement makes one-by-one correlations between the states of the register and the outputs of the divide-by-two counters. A trigger circuit receives the gate outputs, providing a pulse when triggered by a downgoing edge of a square wave output of the gate array. A counter receives the pulse stream and is gated, in turn, in accordance with the cycle time of the utilization circuit so that the accelerometer data employed therein is representative of the portion of the acceleration occurring during a cycle of the utilization circuit.

Abstract: A method for thermal modeling and updating of bias errors in inertial navigation instrument outputs relies upon piecewise cubic spline encoding of data. The temperature range of the thermal model is divided into contiguous intervals of equal length. Instrument bias-versus-temperature data is encoded on an interval-by-interval basis with all intervals normalized so that cubic polynomials of identical form may be fitted between boundary intervals defined by function values and slopes. Instrument bias error at a measured temperature is estimated in the field and an update point (bias, temperature) established. The particular interval is located and the thermal model is re-fit accordingly in the region of the relevant interval. The spline parameters are then adjusted to accommodate the estimated bias error thereby updating the instrument thermal model of bias.

Abstract: An amplifier topology for receiving signals output from a fiber optic rotation sensor and producing voltages that may be processed to determine the rotation rate includes a photodiode for receiving an optical signal and producing a corresponding electrical photodiode output signal. An ultra low noise and ultra low capacitance differential input stage is connected to receive the photodiode output signal. An operational amplifier having low noise and ultra-wide bandwidth is connected to the ultra low capacitance differential input stage to receive the output signal therefrom as a driving signal and to produce a low noise output signal. The differential input stage comprises a first amplifier circuit that includes a first transistor connected to the photodetector to act as a first buffer having low noise, low capacitance and unity gain.

Abstract: Disclosed herein is a Ring Laser Gyroscope Enhanced Resolution System comprising, in a preferred embodiment, a sampling frequency modulation phase-locked loop (or "chirp") which is used in combination with the fast filter. Enhanced resolution is achieved by using the fast filter (preferably a moving average digital filter) to gather optically integrated rate data and accumulate this data. During the data gathering process, the accumulated data is sampled at a rate set by a frequency modulated ("chirper") phase-locked loop in order to remove undesirable data due to aliasing effects upon the digital filter. In this manner, an enhanced resolution of the gyroscope angular measurement is achieved.

Abstract: Sculling error correction means, using third or higher order correction to correct for sculling errors in apparatus for converting angle measuring and acceleration measuring signals from a vehicle set of coordinates into a stabilized set of navigation coordinates.

Abstract: Dispersion in a multi-oscillator cavity is equalized by processing information extracted from the laser beams within the cavity to generate an error signal representative of dispersion bias, and applying a bias control signal to the cavity in such a way as to null the error signal. The bias control signal is preferably a magnetic field applied to the gain medium of the cavity by passing a corresponding current through a coil surrounding the medium. In exemplary embodiments, the laser beams are modulated by modulating the applied magnetic field and/or the path length of the cavity.

Abstract: A triaxial gyro system includes a loop controller for generating a multiplexed digital biasing waveform. The controller receives the multiplexed outputs of the three gyros in digital form from an analog-to-digital converter. The loop controller is arranged to generate a digital waveform for multiplexed driving of the phase modulators associated with the three gyros. Within the loop controller, the multiplexed gyro outputs are independently processed in such a way that a biasing waveform is generated which permits sequential driving of the phase modulators without any need for resetting buffer cycles.

Abstract: A method and apparatus for controlling dither in a multiple gyro system. Each gyro is driven at a distinct nominal frequency. The resulting gyro motions are measured, then demodulated to determine their cross-axis frequency content. Virtual drive signals are generated in light of the cross-axis interactions, then combined with the nominal drive frequencies in such a way that the measured cross-axis effects, resulting in either signal intermodulation or block coning, are nulled.

Abstract: A dithered ring laser gyroscope is mounted upon a support, and a pickoff is positioned between the gyro and the support to produce signals representative of rotation angle between the support and the gyro. A correction means is used to subtract out the effect of the dither from the output signal to the gyro. The correction means includes feedback means in the correction-producing apparatus. That feedback means includes tracking compensation for both gain and phase variation of the pickoff and pickoff electronics. To prevent constant or slowly varying signals from affecting the operation of the apparatus, the invention uses a differentiator or a high pass filter between the compensated output signal and the feedback loops.

Abstract: Disclosed herein is a Digitally Controlled Cavity Length Control System comprising, in a preferred embodiment, an adaptive servo-loop for cavity length, modulation depth and amplitude control, and of the multioscillator ring laser gyroscope; thereby, permitting the simultaneous control of these elements. The cavity length control of the multioscillator, by including a variable RF detector gain, allows the operation of the gyroscope in a slightly detuned fashion to minimize magnetic sensitivity. In this manner, the multioscillator gyroscope may be maintained at a stable operating point suitable for modeling.